The present invention generally relates to filled fluoropolymer compositions. In particular, the present invention relates to microsphere-filled polytetrafluoroethylene (PTFE) compositions, and methods of forming articles from microsphere-filled PTFE compositions.
PTFE is a fluoropolymer that exhibits good chemical resistance, a low coefficient of friction, a low dielectric constant, a broad service temperature range, and good tensile strengths. As such, PTFE is suitable for use in a variety of industrial applications, such as sealant and gasket materials. PTFE exhibits a high molecular weight (e.g., about 1.0×106 grams/mol to about 1.0×108 grams/mol), and demonstrates a high melt viscosity (about 1.0×1011 Poise) above its initial crystalline melting point of about 342° C. The subsequent melting point (after initial melting and cooling) is about 327° C. Due to these characteristics, PTFE is generally non-melt processable and conventional thermoplastic processing methods may not be employed. Therefore, alternative techniques are required to process PTFE, which generally define the grade of PTFE.
PTFE materials suitable for use as gasket materials typically come in two grades: fine-powder-grade PTFE and granular-grade PTFE. Fine-powder-grade PTFE and its derivate compounds generally require the use of paste extrusion techniques to create a sheet form from which specific gasket geometries may be stamped or cut. Paste extrusion involves specific, expensive machinery and volatile solvents in order to process the fine-powder-grade PTFE. Moreover, fine-powder-grade PTFE is typically calendared to densify the produced sheet, which constitutes an additional processing step. As such, it is generally expensive and time consuming to manufacture gaskets and other sealants from fine-powder-grade PTFE.
Granular-grade PTFE may also be used as a gasket material. However, the mechanical properties of granular-grade PTFE are generally inferior to the mechanical properties of fine-powder-grade PTFE. Nonetheless, granular-grade PTFE may be processed by compression molding and skiving of large billets, or cylinders, which is less expensive than paste extrusion. In contrast, fine-powder-grade PTFE is generally not processsable by compression molding, and is limited to the more expensive paste extrusion process.
Fine-powder-grade PTFE and granular-grade PTFE each exhibit several limitations, such as poor dimensional stability and high compression ratios. These factors typically result in low yields and reduced product lives. Filler materials are typically used to increase the dimensional stability and reduce the compression ratios. However, filler materials generally reduce the tensile strengths of the produced gaskets, which correspondingly reduces the resistance of the produced gaskets against tearing or other shear force. As such, there is a need for a PTFE composition that exhibits good dimensional stability, low compression ratios, good tensile strengths, and is inexpensive to produce.